Cathode material



Allg. 17, 1954 Q H, THOMAS 2,686,735

CATHODE MATERIAL Filed Jan. 3, 1951 MFT/1L ELECTRODES 056145550 AND 7055PUMPE!) 007 INVENTOR CHARLES H. THOMAS Patented Aug. 17, 1954 UNITEDSTATES CATHODE MATERIAL Charles Hastings Thomas, Lancaster, Pa.,assignor to Radio Corporation of America, a corporation of DelawareApplication January 3, 1951, Serial No. 204,149

4 Claims.

My invention relates to a directly heated cathode structure, and moreparticularly to a filamentary type cathode for a gas discharge tube.

One type of thyratron gas tube employs a lamentary cathode electrode, ananode electrode and a control electrode or grid mounted between theanode and cathode electrodes. In operation, a difference of potential isestablished between the anode and cathode electrodes to produce anelectron discharge therebetween. The tube is filled preferably with amonatomic gas to amplify the primary discharge between the cathode andanode electrodes. The average amount of current conducted by this typeof tube can be varied within limits by the control grid electrode. Anegative bias, more negative than a critical value, placed upon thecontrol grid will prevent a gaseous discharge in the tube betweencathode and anode. If the potential of the grid electrode is shifted sothat it becomes more positive than a critical value, an electronsufncient toy cause gas ionization. Under this condition the tube breaksdown and there is current ow in the form of a gas discharge between thecathode and anode electrodes. At this point, the control electrode losescontrol of the tube and cannot stop the `discharge even if the gridpotential is shifted below the critical value. That is, a sufficientlynegatively biased grid electrode can control the starting of the tubedischarge but cannot control the tube discharge, once it has begun. Inthe usual application of this type of tube, a bias, below the criticalvalue is normally maintained on the grid electrode. Means are providedto shift the grid bias above the critical value to permit tubeconduction at any scheduled time.

In the type of gas control tube described above, the cathodeconventionally comprises a filament coated with an electron emissivematerial which will emit a copious amount of electrons when heated by anappropriate current owing therethrough. One type of cathode coatingmaterial, which has been used to a large extent to act as' a source ofelectrons, is a mixture of barium and strontium carbonates. However, Ihave found that a cathode coated with such a material does not haveuniform emission over its whole surface due to the varying resistance ofthe coating from one portion to another of the cathode surface. Due tothis fact, there occurs, during tube opiiTENT OFFICE eration, a localheating within the coating which results in a sputtering or sparking of'the cathode material when the current from the cathode is increasedbeyond certain limits. Due to such sparking, a bare area appears on thecathode and, if such a condition continues, the coating will be sparkedfrom the cathode resulting in a short life for the tube. Such sparkingor arcing, furthermore, causes a vaporization of the coating material,which will tend to condense on other elements of the tube such as thegrid electrode, for example. The presence of cathode material on thegrid will induce emission from the grid in a manner and the grid willlose controi of the discharge, during tube operation.

It is thus an object of this invention to provide an improved cathodecoating for a gaseous discharge tube.

It is another object of this invention. to` provide an improved cathodehaving a semi-conductive cathode coating to eliminate local heating ofthe cathode coating.

It is another object of this invention to provide an improved cathodestructure having a coating which, during operation, will produce aminimum of vaporization of the coating material,

It is a further object of my invention to provide a cathode structurehaving an improved coating to provide a copious source of electrons andwhich will operate at a relatively low and an efficient temperature.

One form of my invention is the provision of a thermionic ernissivecoating for a cathode, which provides a relatively low resistance paththrough the coating, and also, which may be sintered to the cathode baseto,l provide good conductivity between the cathode electrode and theeinissive surface of the cathode coating. This invention proposes theuse of a mixture of barium carbonate and nickel oxide which is firstcoated over the surface of a nickel cathode structure. The nickelcathode, with its coating, is fired in a hydrogen atmosphere to `reducethe nickel oxide to a nickel metal and also to sinter a portion of thenickel formed to the metal surface of the cathode structure. After thisring, the coated cathode member is mounted within the tube and furtheractivated to produce the desired emission characteristics.

The novel features which I believe to be char- 4ing through the tube.

assente 3 acteristic of my invention are set forth with particularity inthe appended claims, but the invention itself will best be understood byreference to the following description taken in connection with theaccompanying drawing, in which:

Figure 1 is a sectional View of a gas control tube incorporating acathode made according to my invention.

Figure 2 is a chart showing the steps in the method used for forming thecathode according to my invention.

The gas control tube comprises an envelope lll enclosing a plurality ofelectrodes. An anode electrode I 2 in the shape of a circular platestructure is supported from the top of the tube envelope by a dependingpress structure iii. Appropriate leads I5 pass through the stem or pressi4 and through the glass envelope l@ to an anode terminal Contact lil.The cathode i8 is constructed as a corrugated ribbon bent in a circularspiral and is supported by the edges of the ribbon on a ceramic rod i9,which is supported by rods mounted from the glass press 22 of theenvelope, as shown in Fig. 1. The ends of the lamentary cathode i8 arexed, such as by welding, at 2| to support rods 20, which are connectedin turn, to respective metal base pins 42.

The cathode ribbon i8 is covered, with an electron emitting material.Surrounding the rllamentary cathode it is a heat shield 2d, which isprincipally a metal cylinder mounted coaxially with the circular anodei2. The heat shield 2li may be closed at the bottom by a circular metalplate and at the upper end by a metal plate i6 having an aperture ZS atits center. Aperture 23 is preferably a circular opening coaxial withcylinder 21S and anode i2. The opening 2B provides a path for adischarge between the filamentary cathode it and the anode electrode i2.rEhe purpose of the heat shield structure Eri is mainly to prevent theescape by radiation of heat from cathode i8 and to maintain the heat inthe localized space around the cathode. This permits the use of asmaller filament current. Furthermore, shields 25 and 23 prevent largedissipation of heat to the other electrodes of the tube prevents thesetube parts from operating at an excessive temperature during tubeoperation.

ltiounted coaxially and in alignment with the heat shield aperture 23and the cathode plate i2, and between the anode electrode i2 and thefilamentary cathode lil, is a grid structure 3@ Vfor controlling thedischarge between the anode and cathode electrodes.

An appropriate current passed through the lamentary cathode is heats itto a temperature at which there is a copious emission of electrons fromthe cathode surface. When a difference of potential is establishedbetween the anode i2 and the cathode i8 in a manner such that the anodei2 is positive and the cathode is negative, there will be an electronflow from the cathcde it to the anode i2. In tubes of this type, it isdesirable to amplify this electron dis charge by providing a gaseousmedium within the tube. After tube evacuation, a monatomic gas isadmitted to the tube under a predetermined pressure. The pressure of thegas within the tube is critical and is usually of the order of 50 to 300microns for the rare gases, as krypton, xenon, or argon. The electrondischarge between the cathode lil and anode i2 ionizes the gaseousmedium to increase the amount of current flow- This electrical dischargeCFI 4 between the cathode IS and anode l2 is confined to a path passingthrough aperture 28 and the control grid 3d.

My invention is in the cathode i8 of the tube shown in Figure 1. Onetype of cathode coating commonly used in tubes of this type is a mixtureof barium and strontium carbonates, which upon activation, are changedto barium oxides and strontium oxides. However, I have found that such abarium strontium oxide coating possesses a high resistance between themetal cathode support and the emissive surface of the cathode coating.coating produces local hot spots in the coating, which initiate arcingor sputtering between the cathode and the anode electrodes. This arcingor sputtering has the disadvantage of evaporating the cathode materialaway from these portions of the cathode surface. This consequentlyshortens the life of the cathode electrode. Furthermore the cathodematerial, vaporiaed by the arc, tends to condense on the colderelectrodes of the tube, such as grid til and anode i2 for example.Cathode material on these cold electrodes will cause primary emissionfrom these electrodes when they become heated during operation. Primaryemission from the grid structure 3d, tends to change the controlpotential of the grid, so that it loses control of lthe tube.

A novel type of cathode coating, made according to my invention,eliminates the defects described above. Such a cathode coating is oneconsisting of a mixture of barium carbonate and nickel oxide. Thecathode coating mixture is made of 60% by weight of barium carbonate(BaCOs) and 40% by weight of nickel oxide (NizOa). This mixture issuspended in an ordinary nitrocellulose binder and then ball-milled forsome 20 hours, to mix the components thoroughly and to reduce theparticle size of the barium carbonate. The cathode ribbon i3 ispreferably either nickel or a nickel alloy. Any other appropriate metalmay be used, such as a nickelcobalt alloy, tungsten, tantalum, etc. Thebarium carbonate-nickel oxide coating is sprayed onto the cathode ribboni8 to a weight of between 5 to 8 milligrams per square centimeter. Upondrying, the sprayed coating is held to the surface of the nickel spirali8 by the nitrocellulose binder. The sprayed cathode is next heated orfired in a hydrogen furnace at 1000 C. for 10 minutes. The atmosphere ofthe furnace may be either pure hydrogen or any gas having a reducingaction such as forming gas, which is essentially nitrogen and 10%hydrogen. The reducing atmosphere of the hydrogen furnace tends toreduce some of the nickel oxide of the coating to nickel. Also thefiring of the cathode will sinter some of the nickel particles to thesurface of the base metal of the cathode member I8. Optimum results areobtained when the temperature of the furnace is maintained at around1000 C. A proportionately longer heating time at a lower temperature maybe used. However, the use of a temperature, which is above 1l00 C. willprovide a cathode having a poor electron emission, unless the hydrogenor reducing gas is dry and then the reducing and sintering temperaturemay be as high as 1150 C.

The coated cathode, which has been redY in the hydrogen furnace, asdescribed above, is mounted within the tube i0, in any manner, andpreferably as described in Figure 1. The tube envelope IQ is thenexhausted and the tube baked Such a high resistive Y sansa-'zas ataround 450,"C: for approximately. Al0` minutes.

temperature of\- the cathodeb to 1000 C. This r tends to break-down thecarbonates of the coat- ;:ina1 :not 1broken dawnN byythepprevious iring. Alsoithis heating step drives off the b oxygen formedbvthebreaking down-of the oxides, present. In-,this process,` thetemperatureoithe cathode- Aiswvgradually `increased-b to 107591,(2.,over aA periodof minutes,to.;ccmp1ete the activantionfof the cathode.JI'he` gasses, whichare given .of, are removed byA pumpnaidiirinctbiareriod cfg time. The heating ofeiihecathodeiisbontinued until thereis a residual pressure of a micron or less of mercury in the tube. Also,during this period of time, the metal electrode portions of the tubesuch as the shield 24 and the grid portions 32 and 34, as well as theanode I2 are inductively heated to drive 01T the occluded gasses inthese metal parts.

After the preceding steps, an inert gas, such as zenon for example, isintroduced into the tube to a pressure between 50 to 200 microns ofmercury. The cathode lament I8 is again heated to between 850 and 900C., and a voltage is established between the cathode I8 and the anode I2so that a gaseous discharge is set up in the tube and an anode current`of approximately 3 amperes is drawn for about 3 seconds. This activatesthe cathode by reducing the oxides partially to their metals. The gas isnext pumped out and again refilled with theinert gas and the abovedescribed discharge established again` This is repeated a second time toflush the oxygen from the tube, after which the tube is cooled and thegetters 42fiashed. The tube is then filled to between 90 and 100 micronsof xenon and sealed olf for use. A base 40 is then xed to the tubeenvelope in the well known manner to make suitable contacts 42 with theelectrodes within the envelope. Figure 2 discloses the steps forprocessing such a tube. i

During tube operation, when the cathode is heated to an voperativetemperature of between 800 C. and 850 C. an electron emission will takeplace. This electron emission will be increased by tube operation atnormal filament voltages due to the additional activation of the cathodecoating by positive ion bombardment.

The novel barium oxide nickel coating materially reduces the sparking orarcing that takes place on the surface of the cathode. This isapparently due to the presence of the nickel metal within the coating,which reduces the electrical resistance of the coating material andhence eliminates any local heating of the coating. Furthermore, it isbelieved that the rate of evaporation of the barium metal from thecoating is considerably less than that from a conventional barium oxidestrontium oxide coating. This is evidenced by the fact, that there iswithin the tube less evidence of primary grid emission which can beattributed to condensed cathode material on the grid electrode. therbelieved that the sintering of the nickel, formed in the coating to themetal cathode support, provides the distinct advantages observed. Thesintering provides a direct contact between the coating and the metalcathode support, so that, as described above, the internal resistance ofthe cathode coating is materially reduced with the reduction in arcing.

The process of making my novel cathode coatying described inbox/e,involves the 1 @use i rof 60% barium carbonate; and l,40%. rnickelcxide.material in the original cathode,coatingmixture This is ture. fHowever, 4I ,1 4have `found that `it is not l necessary` to` be limitedgto; these; relative; proportions of` bariumcoxde andnickel oxide. The

, nickelA oxide `maybevaried between 25% and `50 Aof the-mixture.

Also,` I have 4found that following the .teachings Lof my invention,asuccessiully operated cathode can be made by coatingthe nickel`coilsup-1 port with a coating; mixture comprising 40% nickel .oxide andt60% of a.barium b, strontium ,carbonate ,mixtureinstead of ,the bariumcarbonate and in which the same processing steps described above areused. Successfully operated cathcdes have also been made with coatingscomposed of 10% nickel oxide and 90% of a mixture composed ofapproximately 4 parts barium carbonate, 5.5 parts strontium carbonate,and 5 parts of calcium carbonate. I have also found that the cathodecoating of nickel oxide can be replaced with a mixture of bariumcarlbonate and nickel powder. However, it has been found that the lasttwo coatings described did not produce optimum emission.

It is believed that the favorable results produced by the novel cathodedescribed above is due mainly to the dispersal within the coating of thenickel metal, as well as to the sintering of the nickel particles to thecathode support. A photomicrograph of a section of a nickel base cathodecoated with 40% nickel oxide and 60% barium carbonate, processed asdescribed above, indicates that the cathode coating consists ofparticles of nickel interspersed with the barium component and sinteredto the surface of the nickel base cathode.

While certain specic embodiments have been illustrated and described, itwill be understood that various changes and modifications may be madetherein without departing from the spirit and scope of the invention.

What I claim is:

1. The method of preparing a cathode electrode for a discharge device,said method including the steps of, coating a metal base member with amixture of barium carbonate and nickel oxide, the amount of nickel oxidebeing from 25% to 50% of the mixture, heating said coated base member ina hydrogen atmosphere to the sintering temperature of nickel to reducethe nickel oxide of the coating to nickel and to sinter part of thenickel formed to the metal base member.

2. The method of preparing the cathode electrode for a discharge device,said method including the steps of, coating a metal base mem ber with amixture of barium oxide and nickel oxide, the amount of nickel oxidebeing from 25% to 50% of the mixture, heating the coating in a reducingatmosphere to the sintering temperature of nickel to reduce the nickeloxide of the coating to nickel and to sinter to the metal base membersome of the nickel formed.

3. The method of preparing a cathode electrode for a discharge device,said method including the steps of, coating a metal base member with amixture of nickel oxide and at least one of the oxides of barium,strontium, and calcium, the amount of the nickel oxide being from 25% to50% of the mixture, heating the coating in a reducing atmosphere to thesintering temperature of nickel to reduce the nickel 7 oxide of thecoating to nickel and to sinter the nickel formed to the metal basemember.

4. The method of preparing a cathode electrode for a discharge device,said method including the steps of, coating a metal base member with amixture of nickel oxide and at least one of the oxides of barium,strontium and calcium, the amount of the nickel oxide being from 25% to50% of the mixture, heating said coated base member to a temperature ofapproximately 1000o C. in a reducing atmosphere to reduce some of thenickel oxide of the coating to nickel and to sinter at least some of thenickel metal formed to the base member, and heating said coated metalbase member to a temperature between 8 1000 C. and 1100 C. in vacuum toactivate the cathode coating.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,755,257 Holden Apr. 22, 1930 1,809,229 Bartlett et al. June9, 1931 1,903,144 Spanner Mar. 28, 1933 1,935,939V Case i Nov. 21, 19331,983,668 Jones et a1 Dec. 11, 1934 2,041,802 Wilsonet al May 26, 19362,049,372 Hamada et al July 28, 1936 2,103,033 Inman Dec. 21, 19372,142,331

Prescott Jan. 3, 1939

1. THE METHOD OF PREPARING A CATHODE ELECTRODE FOR A DISCHARGE DEVICE,SAID METHOD INCLUDING THE STEPS OF, COATING A METAL BASE MEMBER WITH AMIXTURE OF BARIUM CARBONATE AND NICKEL OXIDE, THE AMOUNT OF NICKEL OXIDEBEING FROM 25% TO 50% OF THE MIXTURE, HEATING SAID COATED BASE MEMBER INA HYDROGEN ATMOSPHERE TO THE SINTERING TEMPERATURE OF NICKEL TO REDUCETHE NICKEL OXIDE OF THE COATING TO NICKEL AND TO SINTER PART OF THENICKEL FORMED TO THE METAL BASE MEMBER.